A self-releasing ratchet mechanism is a particularly desirable adjustment device for a vehicle seat element because of its ease and simplicity of operation. For example, if such a mechanism controls the position of fore-and-aft tilting of a back rest, the seat occupant need only reach over his shoulder and pull the back rest forward until its attitude is satisfactory. If an attempted adjustment tilts it too far to the front, it need only be swung all the way to the front limit of its adjusting motion, whereupon it can be swung freely to the rear limit of such motion for forward adjustment through its whole range. Thus the seat element simply responds to direct actuation, and therefore the seat is more likely to be adjusted to the occupant's comfort than would be the case with a knob or lever that the occupant must first find and then fiddle with. Easy adjustment of a vehicle operator's seat is more than a mere matter of convenience; the operator should be comfortable for safe operation of the vehicle.
Numerous self-releasing ratchet mechanisms are known that are intended for controlling positioning of an adjustable element of a seat, but each of these has some disadvantage that makes it unsuitable for certain vehicle seat applications. In particular, no fully satisfactory mechanism of this type has been devised for an installation in which a seat element that is to be controlled is adjustingly tiltable about a relatively fixed axis extending from side to side of the seat, and wherein it is important that the mechanism be as compact as possible in the directions of adjusting motion of that seat element, that it not rattle under relatively severe vibration and that it not be damaged or thrown out of a selected position of adjustment by an abrupt acceleration in any direction.
The prior patents discussed below disclose self-releasing ratchet mechanisms that were satisfactory for the purposes for which they were intended but which had disadvantages for an installation of the type just described.
U.S. Pat. No. 4,307,913 to Speigelhoff, which is assigned to the assignee of this application, discloses a self-releasing ratchet mechanism that provides for vertical adjustment of a horizontally elongated arm rest of a vehicle seat. It comprises a pawl member, a ratchet member and a latch member, each of which is movable relative to the seat frame and the arm rest as well as relative to the other members, so that there are numerous moving parts. The mechanism is relatively large in the directions of adjusting motions of the arm rest and is therefore not suitable where there is limited space in the adjustment directions.
A self-releasing ratchet mechanism for a back rest that is edgewise vertically adjustable is disclosed in U.S. patent application Ser. No. 774,447, filed Sept. 10, 1985, now U.S. Pat. No. 4,639,039, which has a common assignee with this application. That mechanism is relatively very slender in the front-to-back direction, but it is suitable only for an edgewise adjustable element and not suitable for controlling a backrest that is adjustably tiltable fore-and-aft or a seat pan that is tiltable up and down.
U.S. Pat. Nos. 4,451,084 to Seeley and 4,370,898 to Maruyama disclose self-releasing ratchet mechanisms that similarly lack compactness in the adjusting directions. In each of these, the pawl that cooperates with the position-defining ratchet is connected with an overcenter tension spring that tends to hold the pawl in each of an operative position and an inoperative position between which it can be alternatively flipped with a toggling action. This spring necessarily extends lengthwise in substantially the directions of adjustment so that the mechanism is not well suited for a tiltable back rest or seat pan structure that is shallow in the directions of its adjusting motion.
The relatively old U.S. Pat. Nos. 773,720 to Drury and 970,635 to Matthhews, disclose self-releasing ratchet mechanisms for controlling swinging adjustment of a seat element. Although fairly compact and simple, these mechanisms are suitable essentially only for the articles of furniture (couches and cnairs) for which they are expressly intended, and they would not be satisfactory for a vehicle seat, which must securely maintain its position of adjustment under vibration that may be severe and under widely and abruptly varying loads.
The mechanism disclosed by Matthes, like some of the other prior self-releasing ratchet mechanisms, has the important disadvantage that it cannot be readily adjusted through its full range by a person who remains seated in the seat in which it is incorporated. The reason for this is that after the adjustable element of the seat has been brought to its last defined position of adjustment in the direction in which the engaged pawl and ratchet permit movement of that element, that element must be moved through a further, rather substantial distance in the same direction to bring it to a limit position at which the pawl and ratchet are latched out of engagement with one another to permit free movement of that element in the opposite direction. Thus, if such a prior mechanism controls adjustment of a tiltingly adjustable seat pan, a person sitting on that seat pan who has brought it to its highest position of adjustment, and who wants to release it for adjustment to a lower position, must further raise it to a substantial extent, in awkward opposition to his own weight, in order to release it for lowering.
The arrangement disclosed by Drury illustrates another feature of some prior self-releasing ratchet mechanisms that made them unsuitable for vehicle seats. In that arrangement friction was relied upon to constrain a latch member to move with the adjustable seat element between one position in which the latch member captivated the pawl to maintain it temporarily clear of the ratchet and another position in which the latch released the pawl for re-engagement with the ratchet. The frictional connection provided for a certain amount of relative motion between the frictionally connected parts, needed for effecting captivation and release of the pawl. A vehicle seat mechanism is subjected to vibration and other forces and tends to be exposed to dust and dirt; therefore a frictional connection between two parts of a vehicle seat is an inherently unreliable expedient for constraining one of those parts to move with the other at some times but permitting it to move relative to the other at other times.
In the arrangements disclosed in most of the above mentioned prior patents, an extension spring was employed for maintaining the pawl member and the ratchet member of the pawl and ratchet mechanism in biased engagement with one another. Such a spring has hooks at its opposite ends, one of which is connected to the biased member of the pawl and ratchet mechanism and the other of which is connected to an element of the seat structure. A sharp blow against the seat structure can disconnect one of the spring hooks, necessitating a certain amount of disassembly of the seat for access to the spring so that its detached hook can be reconnected. As indicated above, an extension spring often has the further disadvantage that it must extend lengthwise in a direction in which compactness is particularly desired.